This invention relates to hard facing chromium-base alloys which have a high degree of toughness, wear resistance and corrosion resistance, and pow3ders of the chromium-base alloys which have good weldability for hard facing. This invention also relates to automobile engine valves provided with a hard facing layer of the alloys of the invention, which have a high degree of wear resistance and corrosion resistance.
There are known various wear- and corrosion-resistant hard facing materials such as stellite and other cobalt-chromium-tungsten alloys (to be referred to as Co-Cr alloys), and colmonoy and other nickel-chromium-boron-silicon alloys (to be referred to as Ni-Cr alloys). These alloys are used for hard facing various kinds of structures or machine parts which are subjected to different conditions of use. In recent years the environment in which they are used has become so severe that the wear resistance and corrosion resistance of the known alloys have become insufficient in many applications, and there has been an increasing demand for hard facing materials which have toughness, wear resistance, corrosion resistance and other properties higher than those of Co-Cr or Ni-Cr alloys.
With recent increasing use of high-energy sources such as laser or plasma for hard facing, there has also been a demand for hard facing materials having a high degree of toughness, that is, less susceptible to cracks or fissures which would occur in the hard facing layer in rapid heating and cooling in the hard facing process. With respect to toughness, Co-Cr alloys may be satisfactory with an impact value of 1.0 kgf-m/cm.sup.2. Ni-Cr alloys, however, are poor in toughness with an impact value of 0.15 to 0.2 kgf-m/cm.sup.2, so that cracks may occur in the hard facing layer of the alloys in objects of large sizes or particular shapes.
Japanese unexamined patent application No. 56-9348 discloses a malleable, highly heat-resistant alloy consisting of 10 to 25% by weight of chromium and 10 to 25% by weight of tungsten, the balance being nickel. The alloy has a disadvantage that it is low in hardness and wear resistance.
In an effort to solve the above problems of Co-Cr and Ni-Cr alloys and satisfy the demand for better hard facing materials, the present inventors have conducted various studies and experiments for producing alloys having a high degree of toughness, wear resistance and corrosion resistance, and found that by increasing the amount of chromium in Cr-Ni-W alloy it is possible to increase the hardness of the alloy, and that if molybdenum is added to or substituted for tungsten, the resulting alloy has similar characteristics, and invented hard facing chromium-base alloys which are superior in toughness, wear resistance and corrosion resistance.
Studies and experiments have also been conducted for Cr-Ni-W alloys which can be used in the form of powder for hard facing by plasma or laser welding without deterioration of the shape of the bead formed on the hard facing layer or formation of blowholes in the layer. It has been found out that by adding, if necessary, to the alloy powder one or more of aluminum, yttrium, misch metal, titanium, zirconium and hafnium, and/or by limiting, if necessary, the amount of oxygen contained in the alloy powder, it is possible to certainly prevent formation of blowholes and suppress sputtering which would otherwise be caused under certain conditions of hard facing thereby to improve the shape of the bead formed on the hard facing layer.